In a process of manufacturing a silicon substrate for a semiconductor device, the substrate is thinned according to a thinner shape of the semiconductor device. This thinning process is performed, subsequently to forming a circuit pattern on the silicon substrate, by mechanically polishing a back side opposite to the circuit-formed side. The mechanical polishing entails forming a stressed layer including a micro-crack at the surface of the silicon substrate. In order that strength of the silicon substrate is prevented from decrease due to this stressed layer, etching is performed to remove the stressed layer after the mechanical polishing. For this etching, plasma etching is utilized in place of conventional wet etching with chemical solution.
In order to achieve a higher etching rate, this plasma etching directed to silicon employs gas including carbon tetrafluoride gas as plasma-generating gas. The carbon tetrafluoride gas is ionized or excited by plasma discharge to form an ion and radical which in turn etch the silicon surface.
In the plasma etching using carbon tetrafluoride, a carbon-containing film, i.e., compound of fluorine and carbon, which is a reaction product generated in the plasma processing for the silicon surface, re-adheres to a portion of the silicon surface. This re-adhering compound prevents the plasma etching from being proceeded thus reducing an overall etching rate and varying plasma etching effect according to the degree of the re-adhesion. Consequently, the etched surface has a hazy appearance and hence has reduced visual quality.
This hazy appearance prevents the presence of a fine flaw or the like from being checked visually after the etching, thus impairing quality control. Further, a user who demands a mirror-like appearance qualifies the quality of the appearance of the silicon wafer as a product low.